For reducing a manufacturing time and an environmental problem in a die mold machining, it is preferable that the die mold machining is performed by a numerically-controlled cutting machine tool as much as possible, rather than by an electric discharge machining that requires a long machining time and an electrolyte. However, an increase in a required depth of cut and the consequent increase in an overhang length of a cutting tool make it difficult to perform the cutting operation, thereby increasing dependency on the electric discharge machining. Further, where the tool overhang length is large, a large number of revolutions of the cutting tool is likely to cause a chattering problem. Although the chattering problem could be avoided by reducing the number of revolutions of the cutting tool, the machining would be performed with an efficiency considerably reduced due to a feed rate (i.e., feed amount per time unit) that is necessarily reduced by the reduction in the number of revolutions. Thus, conventionally, there has not been expected a high performance of a cutting operation in a die mold machining.
In recent years, as a measure for solving the above problem, there was developed a radius endmill that has a high rigidity permitting a cutting operation to be performed with a remarkably increased feed amount per tooth. That is, such a radius endmill is capable of performing a cutting operation at a high feed rate even with a relatively low number of revolutions, thereby enabling the cutting operation to exhibit a high performance in the machining efficiency.
In such a conventional radius endmill, as shown in FIG. 9 that is a perspective view showing a major portion of the radius endmill and also FIG. 10 that is a plan and side view showing ridge lines of bottom cutting edges of the endmill, the ridge line 6 of each of the bottom cutting edges 4 extends from a center C of an axial end portion 3 to its free end X at which the ridge line 6 is connected to the ridge line 7 of a corresponding one of the rounded corner cutting edges 5, and is constituted by a straight line.
Further, the ridge line 7 of each of the rounded corner cutting edges 5 extends from the free end X at which the ridge line 6 of a corresponding one of the bottom cutting edges 4 is connected to the ridge line 7 of the each rounded corner cutting edge 5, to a free end Y at which the ridge line 7 of the each rounded corner cutting edge 5 is connected to a ridge line 8 of a corresponding one of peripheral cutting edges 2, and is constituted by a circle of curvature having a convex circular arc shape and a predetermined curvature as seen in a side view that is parallel to an axis of the radius endmill. In the conventional radius endmill, as shown in FIG. 10, centers of curvature of each of the rounded corner cutting edges 5 lie on an imaginary transverse plane CS located in a position that is distant from the free end X by a distance equal to a radius of each of the rounded corner cutting edges 5 in a direction of a rotation axis (in a depth direction) of the radius endmill. The centers of curvature of each rounded corner cutting edge 5 lie on a predetermined portion of an imaginary circle C whose diameter corresponds to a value (=D−2R) that is obtained by subtracting twice the radius (=2R) of each rounded corner cutting edge 5 from the diameter D of the outer peripheral portion 1. For example, as shown in FIG. 10, the centers of curvatures of the ridge line 7 at points a1, a3 lie on respective points b1, b3. That is, the centers of curvatures of the ridge line 7 lie on the predetermined portion of the imaginary circle C (ranging from the point b1 to the point b3).
During the cutting operation, as shown in FIG. 11, each bottom cutting edge 4 is, upon its cutting contact with arbitrary points p1, p2 of a workpiece, moving in a direction perpendicular (orthogonal) to a line passing through the points p1, p2, since the ridge line 6 of each bottom cutting edge 4 is constituted by the straight line, namely, since a radial rake of each bottom cutting edge 4 is neither positive nor negative. Further, in a micro analysis of a cutting action of each cutting edge, each bottom cutting edge 4 and the corresponding rounded corner cutting edge 5 (contiguous to the each bottom cutting edge 4) are substantially concurrently brought into cutting contact with the arbitrary points p1, p2, p3, p4 of the workpiece. In a micro analysis of cutting actions of each adjacent pair of cutting edges, each bottom cutting edge 4 and the corresponding rounded corner cutting edge 5 are in intermittent contact with the workpiece, and intermittently perform the cutting operation.
Thus, since the ridge line 6 of each bottom cutting edge 4 is constituted by the straight line, a length of the ridge line 6 of each bottom cutting edge 4 and a length of the ridge line 7 of each rounded corner cutting edge 5 are made small. Further, as described above, each bottom cutting edge 4 is moving in the perpendicular direction upon its cutting contact with the arbitrary points p1, p2 of the workpiece. Each cutting edge is substantially concurrently brought into contact with the arbitrary points p1, p2, p3, p4 of the workpiece. Each cutting edge is intermittent contact with the workpiece, so as to intermittently perform the cutting operation. Therefore, in the conventional radius endmill in which each bottom cutting edge 4 and each rounded corner cutting edge 5 receive a large load during the cutting operation, there is limitation as to improvement in durability of the cutting tool.
Further, in the conventional radius endmill, where a cutting operation is performed with a considerably increased feed amount per tooth, each of produced chips is considerably thick and heavy, as shown in FIG. 12, whereby an axial component of cutting force acting one the radius endmill is made large. Where such a cutting operation performed with the considerably increased feed amount is a deep milling operation, all the produced chips are not likely to be evacuated from each flute of the endmill, so that some of the produced chips remaining in each flute could interfere each cutting edge and the workpiece, thereby causing risks of breakage of the endmill and defectiveness of a machined surface of the workpiece. Further, for facilitating evacuation of the chips, there is necessity of use of cutting fluid, which could induce an environmental problem. It is noted that there is patent document 1, for example, as a prior art document disclosing the above-described conventional radius endmill.
Patent document 1: JP-2004-141975A